As is well known, chrome-tanning process is a process for converting a hide to a leather by subjecting a raw hide to the so-called beamhouse works such as washing with water, soaking, liming, unhairing, deliming, bating etc. to obtain a green hide which has been prepared in accordance with the kind of the intended leather product, followed by pickling it and letting a chromium salt act upon it.
In the usual chrome-tanning process, a hide after the pickling step is treated in a tanning bath containing a basic chrome complex. In this case, the chrome complex is used in an amount of 2.0-2.5 parts by weight (hereinafter simply referred to as "part(s)") as Cr.sub.2 O.sub.3 per 100 parts of hide, and the tanning is made progress by adjusting pH of the bath to about 3.0 at the beginning, frequently adding a small quantity of alkali such as sodium bicarbonate and thereby adjusting pH to 3.5-4.0 at the end of tanning.
Control of pH of the bath is important, and basicity of chrome salt increases with elevation of pH. At the same time, chrome salt becomes easier to combine with hide. However, if pH is elevated too rapidly, the chrome salt becomes easy to combine with the surface of hide so that an incompletely tanned leather is given. In an extreme case, chromium hydroxide precipitates to make the tanning impossible. Even if it does not occur, olation of chrome salt takes place to form a colloid so that a leather of low quality is given. On the other hand, if tanning progresses at a low pH value, the leather formed is low in the quantity of combined chromium, slender, hard and inferior in quality. As above, the control of pH of bath is quite important for obtaining a leather of good quality by the combination of hide and chrome salt in the tanning bath.
The concentration of chrome salt in tanning bath is also important. In general, as the concentration decreases, the increase in basicity due to elevation of pH becomes remarkable and control of pH becomes more difficult. For example, a high basicity chrome salt having a basicity of 60% is stable in a bath of Cr.sub.2 O.sub.3 50 g/liter, but if this bath is diluted 10 times the basicity of the chrome salt increases to form a precipitate.
Thus, chrome tanning is carried out while elevating the basicity of tanning bath, having a chrome salt concentration of 15-50 g/liter as Cr.sub.2 O.sub.3, from about 30 to about 50. However, chromium in the bath is not always combined to the hide exhaustively but the residual bath after the completion of tanning usually contains 30-50% of the chromium used as uncombined chromium. For the sake of preventing public hazard, the residual liquid is usually subjected to a chemical treatment for removing or recovering the existing chromium, after which it is discharged.
As compared with vegetable tanned leather, chrome-leather is characterized in that it is rich in heat resistance, flexibility and elasticity. On the other hand, its chromium bond is released and it is swollen by an organic acid or an alkali solution, which is regarded as its disadvantage.